Method for treating metal-contaminated water and soil

ABSTRACT

A method for removing metal contaminants from water uses lignin derivatives, such as lignosulfonates and kraft lignin, a coagulant, such as a metal salt, and a pH-increasing composition. The lignin derivative is dispersed in the contaminated water, the coagulant is added and the pH is adjusted as required to cause flocculation. A sludge is formed that contains the metals and that is separated from the treated water by filtration. Related methods are used to reduce the leachable metal content of contaminated soils. The invention also provides a composition for stabilizing the metal contaminants in soil, comprising lignin derivatives, a coagulant and a composition for increasing the pH. The mixture is blended with the contaminated soil, reducing its leachable metal content.

TECHNICAL FIELD

[0001] This invention pertains to the use of lignin derivatives in theremoval of metal contaminants from water. It further pertains to theimmobilization of leachable metal contaminants in soils and sediments.

BACKGROUND OF THE INVENTION

[0002] Contamination of water and soils by heavy metals is a seriousenvironmental concern. As one example, soil contamination by mercury andother heavy metals is a common result of mining operations. Such heavymetals typically leach into water that comes into contact with thecontaminated soil. The metals therefore give rise to environmentalhazards in respect of both the contaminated soil and the watercontaminated thereby, which may find its way into drinking watersupplies.

[0003] Various processes have been proposed for reducing the metalcontent of wastewater. It is known that flocculating agents are of usein such processes. For example, U.S. Pat. No. 5,720,886 (Iwinski)discloses a process for removing metals from mine wastewaters that usesan anionic polymer and a flocculent.

[0004] It is also known that a variety of compounds form complexes withmetal ions, providing the potential to remove the metal ions from thecompositions they contaminate. Lignin is commonly considered to be theprecursor of humic and fulvic acids, which are major organicconstituents of soils. The capacity of humic and fulvic acids to complexmetal ions is well established. For example, U.S. Pat. No. 6,143,692(Sanjay et al.) discloses a process for removing metals from water usinghumic acid.

[0005] Kraft lignin and lignosulfonates are two classes of ligninderivatives available commercially. They are produced as by-products ofthe sulfate and sulfite pulping processes respectively. Lignosulfonateshave been used in processes for the removal of various organiccontaminants from water. Such processes are disclosed, for example, inU.S. Pat. No. 5,736,032 (Cox et al.), U.S. Pat. No. 4,933,087 (Markham,Jr. et al.) and U.S. Pat. No. 5,308,499 (Dixon et al.).

[0006] Lignosulfonates and kraft lignin contain an abundance ofoxygen-containing functional groups, which are capable of forminglignin-metal complexes with high stability through ionic and covalentchemical bonding. It would be desirable to be able to use theseplentiful and inexpensive lignin derivatives in processes for theeffective removal of heavy-metal contaminants from water, and for thestabilization of heavy metal contaminants in soils, so that theirleachability from such soils is substantially reduced.

SUMMARY OF INVENTION

[0007] It is an object of the invention to provide an efficient andeconomical process for the removal of metal contaminants from waterusing lignin derivatives and a coagulant.

[0008] It is a further object of the invention to provide a process forstabilizing the metal contaminants in soils, by use of ligninderivatives and a coagulant, or, alternatively, by use of sludgeproduced by processes for treating water that contains humic or fulvicacids.

[0009] According to one embodiment of the invention, there is provided amethod of removing metal contaminants from water. Lignin derivatives,such as lignosulfonates or kraft lignin, are dispersed in thecontaminated water. A coagulant, such as a metal salt, is added. The pHof the water is adjusted, if necessary, to cause the formation of afloc. For example, the pH may be adjusted by the addition of a suitablecomposition, such as hydrated lime. The floc is allowed to coagulate andform a sludge, which is then separated from the treated water,preferably by filtration. In a variant of this method, the floc isseparated from the treated water directly after its formation, ratherthan allowing it to settle.

[0010] According to a further aspect of the invention, there is provideda method for reducing the leachable metal content of metal-contaminatedsoil. A solid mixture is prepared comprising lignin derivatives, acoagulant and a composition for increasing the pH of the soil, forexample, hydrated lime. This mixture is then blended with thecontaminated soil.

[0011] According to a further embodiment of the invention, there isprovided a method of reducing the leachable metal content ofmetal-contaminated soil using a sludge obtained from a water treatmentprocess, where the water contains humic or fulvic acid and a heavymetal, for example, metal-contaminated groundwater. The water treatmentprocess comprises dispersing a coagulant in the water, adjusting the pHto cause the formation of a floc and allowing the floc to coagulate andform a sludge. The metal-contaminated soil is then blended with thesludge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The water that can be treated by the methods of the invention canbe any water contaminated by metals, for example, wastewater from amining site contaminated by mercury. In this specification, the term“soil” includes soil, sediments, mine tailings, gravel, sand,cementitious materials, etc. The term “mixture” in this specificationmeans any combination of the components in issue, without regard to thephysical form of the combination, and includes dispersions, suspensions,solutions, colloids, etc.

[0013] The metals that can be removed according to the methods of theinvention are heavy metals, including mercury, chromium, copper, lead,nickel and zinc, as well as lighter metals, such as aluminum.

[0014] The invention provides a new water treatment process in whichlignin derivatives are dissolved in metal-contaminated water, where theyform a lignin-metal complex. Following formation of this complex, acoagulant, such as ferric chloride, is added to the water, where ithydrolyses to form a floc, such as ferric floc. This floc absorbs thelignin-metal complex, coagulates and settles as sludge. Thus, a clearfiltrate is formed which contains only very small residual amounts ofthe metal contaminants. For example, the dissolved metal content of thewater can be reduced from about 4,000 ppb to less than 1 ppb.

[0015] In general terms, the method for the removal of metalcontaminants from water comprises the steps of dispersing ligninderivatives in the water, adding a coagulant, while agitating themixture, making flocculation occur, allowing the floc to coagulate andsettle to form a sludge, and dewatering the sludge. The sludge producedin this process traps stable complexes of lignin derivatives and metalcompounds, which are rendered unleachable according to the standard testprotocol, namely the “Toxicity Characteristic Leaching Procedure” (TCLP)(EPA Method 1311).

[0016] The lignin derivatives used are preferably lignosulfonates, kraftlignin and sulfonated kraft lignin salts, and mixtures thereof. Thelignosulfonates include ammonium, calcium, sodium and potassiumlignosulfonates. The sulfonated kraft lignin salts include the ammonium,calcium, sodium and potassium salts. All these lignin derivatives areavailable as by-products of pulp mill processes.

[0017] The coagulants used are any ones that can form a complex with thelignin derivatives that can scavenge the metal ions to be removed. Thecoagulant is preferably a metal salt, including iron salts and aluminumsalts. More preferably it is ferric chloride, ferric sulfate, aluminumchloride or aluminum sulfate.

[0018] The mass ratio of the lignin derivatives to the coagulantsignificantly affects the efficiency of the metal removal process.Preferably, the mass ratio employed is at least 1:0.5 where light metalssuch as aluminum are being removed. For heavy metals the preferred massratio is at least 1:1 and more preferably at least 1:2. For greaterclarity, “at least 1:2” means 1:2 and 1: more than 2.

[0019] The formation of a floc depends on the pH of the aqueous mixture.Where the water being treated is sufficiently basic, the pH after theaddition of lignin derivatives and coagulant may still be sufficientlyhigh that a floc will form. In such case the step of causing theformation of a floc merely involves waiting for the floc to form afterthe lignin derivatives and coagulant have been dispersed in the water.In general, however, it is necessary to increase the pH by the additionof a suitable basic composition. This is preferably done after theaddition of the coagulant. It can also be done before dispersing thelignin derivative in the water to be treated, or after dispersing thelignin derivative but before dispersing the coagulant.

[0020] Suitable compositions for adjusting the pH include hydrated lime,calcium oxide, magnesium hydroxide, soda ash and sodium hydroxide. ThepH is adjusted to a pH appropriate for effecting coagulation, preferablya pH in the range of 4-8, and more preferably in the range of 5-6.

[0021] The sludge is removed by any convenient means. Preferably, it isremoved by filtering the treated water and sludge, for example in afilter press, to separate the sludge, which contains the complexedmetals, from a clear filtrate, having a greatly reduced metal content.Other sludge-removal means include decanting, centrifuging and using aclarifier.

[0022] In a variant of the method described above for removing metalcontaminants from water, rather than allowing the floc to coagulate andsettle as a sludge, once the floc forms it is separated from thesolution, for example by filtration, leaving a clear filtrate. In thismethod, the step of allowing the floc to coagulate and settle is notrequired. The floc is filtered out directly, using a filter that issufficiently fine to remove floc comprising small particles.

EXAMPLE 1

[0023] A wastewater having a pH of 9.3, a total mercury concentration of2490 μg/L, a dissolved mercury concentration (size≦0.45 μm) of 1880μg/L, and a conductivity of 1300 microSiemens/cm, was treated by mixing100 parts by weight of wastewater with 0.4 parts by weight ammoniumlignosulfonates. 3.2 parts by weight of a 40 weight % ferric chloridesolution was added while agitating the mixture. The pH was adjusted to5.1 using hydrated lime. The resulting sludge was removed by filtration,leaving a filtrate having a mercury content of 0.67 μg/L.

EXAMPLE 2

[0024] Following the procedure of Example 1, 100 parts by weight of thewastewater of Example 1 was treated with 0.9 parts by weight of kraftlignin and 4 parts of a 40 weight % ferric chloride solution. The finalpH after hydrated lime addition was 5.2. The filtrate had a mercurycontent of 0.75 μg/L.

[0025] The above Examples 1 and 2 show that lignin derivatives,including ammonium lignosulfonates and kraft lignin, are capable oftrapping dissolved mercury in a ferric sludge. The mass ratio of ligninderivative to ferric chloride in Examples 1 and 2 was 1:3.2 and 1:2.1respectively.

EXAMPLE 3

[0026] Following the procedure of Example 1, 100 parts by weight of thewastewater of Example 1 was treated with 0.4 parts by weight of ammoniumlignosulfonate and 1 part of a 40 weight % ferric chloride solution. Thefinal pH after hydrated lime addition was 5.0. The filtrate had amercury content of 126 μg/L. In this example, the mass ratio of ligninferric chloride was 1/1, which did not achieve the low mercury contentin the filtrate of Examples 1 and 2. A suitable mass ratio of the ligninderivative to ferric chloride is necessary to achieve very low residualconcentrations of mercury in the treated water.

EXAMPLE 4

[0027] A wastewater having a pH of 5.5, a total mercury concentration of3370 μg/L, a dissolved mercury concentration (size≦0.45 μm) of 2657 μg/Land a conductivity of 3100 microSiemens/cm, was treated by mixing 100parts by weight of wastewater and 0.8 parts by weight of a 40 weight %ferric chloride solution giving a final pH of 3.9. The filtrate had amercury content of 1480 μg/L. This example shows that the addition offerric chloride alone is not sufficient to trap mercury to achieve thelow levels of residual mercury obtained in Examples 1 and 2.

EXAMPLE 5

[0028] A wastewater was obtained from a soil washing operation having apH of 11.6, a total mercury concentration of 498 μg/L, a dissolvedmercury concentration (size≦0.45 μm) of 5.63 μg/L, suspended mercuryconcentration (0.45 μm<size≦2.0 μm) of 68 lg/L and a conductivity of 800microSiemens/cm. This wastewater was treated according to the procedureof Example 1 by adding 0.2 parts by weight of ammonium lignosulfonate to100 parts by weight of wastewater, followed by the addition of 3.3 partsby weight of 40 weight % ferric chloride solution and 0.56 parts byweight of hydrated lime. The pH was adjusted to a value of 5.4 throughthe addition of soda ash. The treated wastewater was passed separatelythrough filter paper of pore size 0.45 μm and through filter paper ofpore size 2.0 μm. The dissolved mercury concentration (size≦0.45 μm) ofthe filtrate was 0.61 μg/L and the suspended mercury concentration (0.45μm<size≦2.0 μm) was not detectable. Suspended mercury colloids wereefficiently removed by the process.

EXAMPLE 6

[0029] A simulated wastewater was prepared by blending tap water andstandard solutions of aluminum, chromium, copper, lead, zinc and nickel.The concentrations of the metal ions in the simulated wastewater weredetermined by ICP analysis and are summarized in Table 1. The simulatedwastewater was treated by mixing 100 parts by weight of wastewater and0.2 parts by weight of ammonium lignosulfonate followed by the additionof 3.2 parts by weight of 40 weight % ferric chloride solution. The pHof the solution was adjusted to 5.6 by adding 0.07 parts of sodiumsulfite and hydrated lime. A floc formed which settled as a sludge. Aportion of the solution was passed through a filter paper with a poresize of 0.45 μm and was analyzed by ICP. A second portion of thesolution was treated by adjusting the pH to 8.6 through the addition ofsoda ash. After filtration through a filter paper with a pore size of0.45 μm this second portion was also analyzed by ICP. The analyticalresults are summarized in Table. 1. TABLE 1 Metal Ion Removal fromAqueous Solutions Con- Concentration After Treatment centration (μg/L)Before At pH 5.6 At pH 8.6 Metal Treatment Reduction Reduction Species(μg/L) (μ/L) (%) (μg/L) (%) Aluminum 470 50 89.4 60 87.2 Chromium 213 1095.3 10 95.3 Copper 2850 24 99.2 22 99.2 Lead 120 <MDL⁽¹⁾ >75<MDL⁽¹⁾ >75 Nickel 120 40 81.0 100 52.4 Zinc 7910 43 99.5 <5 >99.9

[0030] The results of Table 1 show that metal ions can be removedeffectively from aqueous solutions through the process of thisinvention. It is also evident that, in addition to the mass ratiobetween lignin derivatives and ferric chloride, final pH is an importantparameter which affects the efficiency of metal recovery fromwastewater.

[0031] According to a second embodiment of the invention, there isprovided a method for reducing the leachable metal content ofmetal-contaminated soil.

[0032] The natural characteristic of soil to stabilize diverse metalions is based on the ability of humic and fulvic acids to form stablecomplexes with polyvalent metal ions, such as Al³⁺, Fe³⁺, Cu²⁺, Pb²⁺,Ca²⁺ and Mn²⁺. The formation of these complexes plays an important rolein the mobilization, transport, segregation and deposition of metals insoils, sedimentary rocks, and biogenic deposits of various types.

[0033] In a similar manner, the addition of small amounts of ligninderivatives to the metal-contaminated soils increases the capacity ofthe soil to retain metal ions. This is believed to be due to theformation of metal complexes with the lignin derivatives through strongcoordinate bonding. While linking together, the negatively charged clayand lignin derivatives are neutralized by the positively charged metalions.

[0034] In general terms, the method for reducing the leachable metalcontent of soil comprises the steps of preparing a mixture of a ligninderivative, a coagulant and a compound for increasing the pH of thesoil, and blending the mixture, which acts as a metal-absorbent, withthe soil to be treated.

[0035] The metals that can be removed, and the lignosulfonatederivatives, pH-adjusting compositions and flocculents that are used inthis method, are the same as those described above in respect of themethod of removing metal contaminants from water.

[0036] The coagulant is provided in the form of an aqueous solution oras a solid powder. Aqueous solutions of coagulant having about 39-45weight percent of coagulant are preferred. Ferric chloride solutionshaving about 40 weight percent ferric chloride in water are commerciallyavailable and are particularly preferred.

[0037] The mass ratio of the lignin derivatives to the composition forincreasing pH is preferably in the range of 1:1 to 1:8, and morepreferably 1:1 to 1:4.

[0038] In a particularly preferred mixture, the pH-adjusting compositionis hydrated lime and the coagulant is an aqueous ferric chloridesolution having about 40 weight percent ferric chloride. The mass ratioof lignin derivatives to hydrated lime to ferric chloride solution isabout 1:1.5:2.5.

[0039] In the blend of mixture and soil being treated, the mass ratio ofmixture to soil is preferably in the range of 1:5 to 1:100.

[0040] The method reduces the leachable mercury content of the soil tosuch an extent that the stabilized soils can, in many jurisdictions, belegally disposed of in non-hazardous waste disposal sites.

[0041] As an illustration of this aspect of the invention, ligninderivatives, ferric chloride and hydrated lime were blended in suitableproportion to produce a solid absorbent. This mixture was blended withmercury-contaminated soil in suitable proportion. The TCLP-leachablemercury of this stabilized soil was typically reduced by over 80%.

EXAMPLE 7

[0042] A mercury-contaminated soil having the characteristics shown inTable 2 from a chloralkali plant site was treated by blending 100 partsby weight of the soil with 10 parts by weight of a sodiumlignosulfonate-ferric chloride matrix having hydrated lime as thepH-increasing component. The mass ratio of lignin derivatives tohydrated lime to ferric chloride was about 1:1.5:2.5. The stabilizedsoil was then tested according to the TCLP procedure at intervals over aperiod of 30 days. The results are set out in Table 2. The leachablemercury content of the soil was reduced by about 80%. TABLE 2Stabilization of Leachable Mercury in Soil Sample Before After TreatmentAnalysis Treatment 15 min 7 days 14 days 21 days 30 days Soil pH⁽¹⁾ 11.811.8 11.4 11.5 11.2 11.0 TCLP⁽²⁾ 6.1 6.4 7.0 6.4 6.4 7.0 extract pHTotal Hg in 268 76.4 35.0 28.0 35.8 39.3 TCLP extract (μg/L) Reduction —71.5 86.9 89.6 86.6 85.3 of Leachable HG (%)

[0043] According to a further embodiment of the invention, there isprovided another method for reducing the leachability of metals inmetal-contaminated soil. This method does not require the addition oflignin derivatives, and makes use of the fact that humic and fulvicacids are naturally present in many groundwaters. A sludge is preparedaccording to a process essentially the same as that described above forremoving contaminants from water, except that lignin derivatives are notused. First, water (such as groundwater) containing humic or fulvic acidand a metal is provided, and a coagulant is dispersed therein. A floc iscaused to be formed, preferably by the addition of a ph-increasingcomposition. The floc is allowed to coagulate and form a sludge which isthen separated from the treated groundwater, for example by filtration.The sludge thus produced is blended with the metal-contaminated soil. Asa variant of this method, the floc is separated directly after itsformation, for example by filtration, and the filtered flow, whichcomprises a sludge, is blended with the metal-contaminated soil.

[0044] In this process, the metals that can be stabilized and thepH-adjusting compositions and flocculents used the same as thosedescribed above in respect of the method of removing metal contaminantsfrom water.

[0045] In the blend of sludge and soil, the mass ratio of sludge to soilis preferably in the range of 1:5 to 1:100.

EXAMPLE 8

[0046] Two large scale trials were carried out where themercury-contaminated soil characterized in Table 2 was stabilized byusing a ferric sludge generated in a groundwater treatment plant. Thisplant was operated to remove humic and fulvic acids and mercury throughflocculation with ferric chloride. In one trial, 5.7 tonnes of soil wereblended with 0.77 tonnes of ferric sludge, while in a second trial 9.1tonnes of soil were blended with 0.92 tonnes ferric sludge. Onceblending of the soil and sludge through an excavator was completed,stabilized soil samples were tested by TCLP. The leachable mercurycontent of the soil was reduced from 245 μg/L to less than 40 μg/L. Inrepeated TCLP tests over a period of 60 days, the leachable mercuryremained in the range of 13.8 μg/L to 38.6 μg/L.

[0047] As will be apparent to those skilled in the art in the light ofthe foregoing disclosure, many alterations and modifications arepossible in the practice of this invention without departing from thespirit or scope thereof. Accordingly, the scope of the invention is tobe construed in accordance with the substance defined by the followingclaims.

What is claimed is:
 1. A method of removing metal contaminants fromwater, comprising the steps of: (a) dispersing lignin derivatives insaid metal-contaminated water; (b) dispersing a coagulant in the mixtureformed in step (a); (c) causing the formation of a floc in the mixtureformed in step (b); (d) allowing said floc to coagulate and form asludge; and (e) separating said sludge from an aqueous filtrate.
 2. Amethod of removing metal contaminants from water, comprising the stepsof: (a) dispersing lignin derivatives in said metal-contaminated water;(b) dispersing a coagulant in the mixture formed in step (a); (c)causing the formation of a floc in the mixture formed in step (b); and(d) separating said floc from an aqueous filtrate.
 3. A method accordingto claim 1, wherein said step of causing the formation of a floccomprises adjusting the pH of said water being treated so as to causeflocculation.
 4. A method according to claim 1 wherein said metalcontaminants comprise one or more of mercury, chromium, copper, lead,nickel, zinc and aluminum.
 5. A method according to claim 1 wherein saidlignin derivatives comprise lignosulfonates.
 6. A method according toclaim 1 wherein said lignin derivatives comprise kraft lignin.
 7. Amethod according to claim 1 wherein said lignin derivatives comprisesulfonated kraft lignin salts.
 8. A method according to claim 4 whereinsaid lignosulfonates comprise one or more of: (i) ammoniumlignosulfonates; (ii) calcium lignosulfonates; (iii) sodiumlignosulfonates; and (iv) potassium lignosulfonates.
 9. A methodaccording to claim 7 wherein said sulfonated kraft lignin salts comprisesulfonated kraft lignin salts of ammonium, calcium, sodium or potassium.10. A method according to claim 1 wherein said coagulant is an ironsalt.
 11. A method according to claim 10 wherein said iron salt isferric chloride.
 12. A method according to claim 10 wherein said ironsalt is ferric sulfate.
 13. A method according to claim 1 wherein saidcoagulant is an aluminum salt.
 14. A method according to claim 13wherein said aluminum salt is aluminum chloride or aluminum sulfate. 15.A method according to claim 1 wherein the mass ratio of said ligninderivatives to said coagulants is at least 1:0.5.
 16. A method accordingto claim 1 wherein the mass ratio of said lignin derivatives to saidcoagulants is at least 1:1.
 17. A method according to claim 1 whereinthe mass ratio of said lignin derivatives to said coagulants is at least1:2.
 18. A method according to claim 3 wherein said pH is adjusted bythe addition of one or more of hydrated lime, calcium oxide, magnesiumhydroxide, soda ash and sodium hydroxide.
 19. A method according toclaim 3 wherein said pH is adjusted to a pH in the range 4-10.
 20. Amethod according to claim 3 wherein said pH is adjusted to a pH in therange of 5-6.
 21. A method according to claim 1 wherein step (d)comprises allowing said floc to settle in a clarifier.
 22. A methodaccording to claim 1 wherein step (e) comprises removing said sludge byfiltration.
 23. A method of removing metal contaminants from water,comprising the steps of: (a) dispersing lignin derivatives in saidmetal-contaminated water, said lignin derivatives comprising one or moreof lignosulfonates and kraft lignin. (b) dispersing ferric chloride inthe mixture formed in step (a), the mass ratio of the lignin derivativesto the ferric chloride being at least 1:1; (c) adjusting the pH of themixture formed in step (b) to a pH in the range of 4-10, causing theformation of a floc; (d) allowing said floc to settle, resulting in aliquid and a ferric sludge; and (e) separating said sludge from saidliquid;
 24. A method of reducing the leachability of metals ofmetal-contaminated soil, comprising the steps of: (a) preparing amixture comprising: (i) lignin derivatives; (ii) a coagulant; and (iii)a composition for increasing the pH of said soil; and (b) blending saidmixture with said soil.
 25. A method according to claim 24 wherein saidmetal contaminants comprise one or more of mercury, chromium, copper,lead, nickel and aluminum.
 26. A method according to claim 24 whereinsaid lignin derivatives comprise lignosulfonates.
 27. A method accordingto claim 24 wherein said lignin derivatives comprise kraft lignin.
 28. Amethod according to claim 24 wherein said lignin derivatives comprisesulfonated kraft lignin salts.
 29. A method according to claim 24wherein said lignosulfonates comprise one or more of: (i) ammoniumlignosulfonates; (ii) calcium lignosulfonates; (iii) sodiumlignosulfonates; and (iv) potassium lignosulfonates.
 30. A methodaccording to claim 28 wherein said sulfonated kraft lignin saltscomprise sulfonated kraft lignin salts of ammonium, calcium, sodium orpotassium.
 31. A method according to claim 24 wherein said coagulant isan iron salt.
 32. A method according to claim 31 wherein said iron saltis ferric chloride.
 33. A method according to claim 31 wherein said ironsalt is ferric sulfate.
 34. A method according to claim 24 wherein saidcoagulant is an aluminum salt.
 35. A method according to claim 34wherein said aluminum salt is aluminum chloride or aluminum sulfate. 36.A method according to claim 24 wherein said composition for increasingthe pH is one or more of hydrated lime, calcium oxide, magnesiumhydroxide, soda ash and sodium hydroxide.
 37. A method according toclaim 24 wherein the mass ratio of said lignin derivative to saidcoagulant is at least 1:0.5.
 38. A method according to claim 24 whereinthe mass ratio of said lignin derivative to said coagulant is at least1:1.
 39. A method according to claim 24 wherein the mass ratio of saidlignin derivatives to said coagulant is at least 1:2.
 40. A methodaccording to claim 24 wherein the mass ratio of said lignin derivativesto said composition for increasing the pH is in the range of 1:1 to 1:4.41. A method according to claim 24 wherein the mass ratio of said ligninderivatives to said composition for increasing the pH is in the range of1:1 to 1:8.
 42. A method according to claim 24 wherein the mass ratio ofsaid mixture to said soil is in the range of 1:5 to 1:100.
 43. A methodof reducing the leachability of mercury from a mercury-contaminatedsoil, comprising the steps of: (a) preparing a mixture comprising: (i) alignin derivative comprising one or more of lignosulfonates and kraftlignin; (ii) an aqueous ferric chloride solution; and (iii) hydratedlime; and (b) blending said mixture with said contaminated soil.
 44. Amethod of reducing the leachability of metals of metal-contaminatedsoil, comprising the steps of: (a) preparing a sludge prepared by theprocess of: (i) providing water containing humic or fulvic acid and ametal; (ii) dispersing a coagulant in said water; (iii) causing theformation of a floc in the mixture of step (a)(ii); (iv) allowing saidfloc to coagulate and form said sludge; and (v) separating said sludgefrom said treated water; and (b) blending said sludge with saidmetal-contaminated soil.
 45. A method of reducing the leachability ofmetals of metal-contaminated soil, comprising the steps of: (a)preparing a sludge prepared by the process of: (i) providing watercontaining humic or fulvic acid and a metal; (ii) dispersing a coagulantin said water; (iii) causing the formation of a floc in the mixture ofstep (a)(ii); and (iv) separating said floc from an aqueous filtrate;and (b) blending said separated floc with said metal-contaminated soil.46. A method according to claim 44 wherein said step of causing theformation of a floc comprises adjusting the pH of said water so as tocause flocculation.
 47. A method according to claim 44 wherein saidmetal contaminants comprise one or more of mercury, chromium, copper,lead, nickel, zinc and aluminum.
 48. A method according to claim 44wherein said coagulant is an iron salt.
 49. A method according to claim48 wherein said iron salt is ferric chloride.
 50. A method according toclaim 48 wherein said iron salt is ferric sulphate.
 51. A methodaccording to claim 44 wherein said coagulant is an aluminum salt.
 52. Amethod according to claim 44 wherein said aluminum salt is aluminumchloride or aluminum sulfate.
 53. A method according to claim 44 whereinstep (d) comprises allowing said floc to settle in a clarifier.
 54. Amethod according to claim 44 step (e) comprises removing said sludge byfiltration.
 55. A method according to claim 46 wherein said pH isadjusted by the addition of one or more of hydrated lime, calcium oxide,magnesium hydroxide, soda ash and calcium hydroxide.
 56. A methodaccording to claim 46 wherein said pH is adjusted to a pH in the rangeof 4-8.
 57. A method according to claim 46 wherein said pH is adjustedto a pH in the range of 5-6.
 58. A method of reducing the leachabilityof mercury from a mercury-contaminated soil, comprising the steps of:(a) providing a sludge prepared by the process of: (i) providinggroundwater containing humic and fulvic acids and mercury; (ii)dispersing ferric chloride in said water; (iii) adjusting the pH of saidwater so as to cause the formation of a floc; (iv) allowing said floc tocoagulate and form said sludge; and (v) separating said sludge from saidtreated water; and (b) blending said sludge with saidmercury-contaminated soil.
 59. A blend of said sludge and said soilprepared by the method of claim
 44. 60. A blend of said sludge and saidsoil prepared by the method of claim
 58. 61. A mixture prepared by themethod of step (a) of claim 24.